Phytase mutant

ABSTRACT

Disclosed is a thermostable phytase, in which at least one pair of introduced disulfide bonds is included in the amino acid sequence of wild-type Escherichia coli phytase or mutant Escherichia coli phytase, and after the introduction, the properties of the phytase can be improved, especially the heat stability, steam stability and granulation stability, which are superior to those of the existing wild-type or mutant phytase; compared to the engineered phytase in which disulfide bonds are introduced, the heat stability is also significantly improved.

BACKGROUND Technical Field

The present invention belongs to the field of protein engineering, and relates to a phytase from gram-negative bacteria, more particularly to an Escherichia coli phytase. After modification through introduction of a pair or a plurality of pairs of disulfide bonds, its heat stability is improved.

Related Art

Phytase, i.e., myo-Inositol hexakisphosphate phosphohydrolase, belongs to orthophosphate monoacetate phosphohydrolase, catalyzes hydrolysis of phytic acid to produce lower myo-Inositol phosphate derivatives and inorganic phosphoric acid, and can hydrolyze the phytic acid into free myo-Inositol under some conditions. The phytic acid is most abundant in seeds of crops such as grains, beans and oil plants, which accounts for up to 1%-3% of the plants and 60%-80% of the total phosphorus content of the plants. However, the phosphorus in the phytic acid cannot be directly absorbed and utilized, and has to be firstly hydrolyzed into inorganic phosphates in the digestive tract. Studies have shown that monogastric animals (such as pigs, chickens, ducks and geese) have a very low utilization rate of the phosphorus in the phytic acid due to the lack of phytase. Moreover, due to strong electronegativity of the phytic acid, the phytic acid generally forms insoluble salts with divalent or trivalent cations, such as Ca²⁺, Zn²⁺ and Fe²⁺, so as to hinder the absorption of minerals by the small intestines. The phytic acid may further form a complex with protein, amino acid, fatty acid and the like to influence their absorption and utilization. The phytic acid may further be combined with pepsin, chymotrypsin, trypsin and the like to reduce the activity of digestive enzymes. Therefore, by adding the phytase into monogastric animal feed, the utilization rate of phosphorus in the animal feed can be improved, the phosphorus content in animal excreta can be reduced, and the energy utilization rate of the protein and the feed can be improved.

The phytase is used as a feed additive to be added into a feed raw material in advance. Through processes such as high-temperature granulation (70-95° C., time: 30 s-120 s), the feed is produced for animal feeding. Therefore, in order to maximize the effect of the phytase, the phytase needs to resist a high temperature, i.e., the phytase should have good heat resistance. Commercially available phytases are mainly derived from Aspergillus niger (as described in U.S. Pat. No. 5,436,156), Escherichia coli (as described in U.S. Pat. No. 7,432,098), Citrobacter braakii (as described in US20100261259), Buttiauxella sp. (as described in U.S. Pat. No. 8,143,046) and the like. These phytases have different acid and heat resistance properties due to their different sources. Nielsen et al (J Agric Food Chem. 2015, 63 (3): 943-50) compared commercial phytase properties, and the result showed that an Escherichia coli phytase showed the best characteristics. The Escherichia coli phytase products described herein are mutants modified by protein engineering, and thus have better heat stability. U.S. Pat. Nos. 8,540,984, 9,765,313, 7,432,098 and 8,877,478 describe mutant enzymes with improved heat stability obtained by random mutation and site-directed mutation screening on the Escherichia coli phytase sequences. Patent Applications No. US20130017185 and US20170240872 mention that the heat stability of the enzyme can also be improved through introduction of specific disulfide bonds according to three-dimensional structures of the Escherichia coli phytase protein.

There is also a need in the art to provide more thermostable phytases.

SUMMARY

The inventor has found that through introduction of a pair or a plurality of pairs of disulfide bonds to specific positions in the amino acid sequence of an Escherichia coli wild-type phytase (such as one with more than 85% of sequence identity with an Escherichia coli wild-type phytase as shown by SEQ ID NO: 1) or an Escherichia coli phytase mutant (such as one with more than 75% of sequence identity with an Escherichia coli phytase mutant as shown by SEQ ID NO: 2), its stability can be improved, so that the objective of the present invention is achieved.

In some implementations, a pair or a plurality of pairs of disulfide bonds as shown in Table 1 are introduced to specific positions in the amino acid sequence of the Escherichia coli phytase mutant with more than 85% of sequence identity with the Escherichia coli wild-type phytase as shown by SEQ ID NO: 1. In some preferred implementations, a pair or a plurality of pairs of disulfide bonds as shown in Table 1 are introduced to specific positions in the amino acid sequence of the Escherichia coli phytase with more than 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% of sequence identity with the Escherichia coli wild-type phytase as shown by SEQ ID NO: 1.

In some other implementations, a pair or a plurality of pairs of disulfide bonds as shown in Table 1 are introduced to specific positions in the amino acid sequence of the Escherichia coli phytase mutant with more than 75% of sequence identity with the Escherichia coli phytase mutant as shown by SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 79 or SEQ ID NO: 99. In some preferred implementations, a pair or a plurality of pairs of disulfide bonds as shown in Table 1 are introduced to specific positions in the amino acid sequence of the Escherichia coli phytase mutant with more than 80%, 82%, 84%, 86%, 88%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% of sequence identity with the Escherichia coli phytase mutant as shown by SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 79 or SEQ ID NO: 99.

In some implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on at least 1 position: 1, 25, 30, 36, 37, 38, 39, 46, 55, 60, 62, 65, 69, 70, 73, 74, 75, 76, 77, 79, 80, 85, 101, 108, 109, 111, 114, 116, 118, 120, 123, 126, 127, 133, 137, 138, 139, 141, 142, 146, 151, 157, 159, 161, 173, 176, 178, 180, 183, 184, 185, 186, 187, 188, 189, 204, 211, 233, 235, 245, 253, 255, 267, 276, 282, 283, 284, 286, 287, 288, 291, 295, 297, 311, 315, 317, 318, 327, 341, 354, 363, 367, 369, 370, 380, 382, 383, 385, 391, 402 and 408. In some preferred implementations, compared to the wild-type Escherichia coil phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on at least 1 position: 25, 46, 62, 70, 73, 74, 75, 76, 114, 137, 142, 146, 159, 173, 204, 255, 282, 283 and 284. In some more preferred implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on at least 1 position: 25, 46, 62, 70, 73, 74, 75, 76, 114, 137, 142, 146, 159, 173, 204 and 255.

In some specific implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on the following positions: 46, 62, 73, 75, 146, 159, 204 and 255. In some other specific implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on the following positions: 25, 46, 62, 70, 73, 75, 114, 137, 142, 146, 159 and 255. In some specific implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on the following positions: 46, 62, 70, 73, 74, 75, 76, 146, 159, 173, 255, 282, 283 and 284. In some other specific implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on the following positions: 25, 46, 62, 70, 73, 74, 75, 114, 137, 142, 146, 159, 173, 255, 282, 283 and 284.

In some implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has at least one of the following mutations: Q1S, Q1V, Q1N, A25F, Q30K, A36K, W37F, P38Y, T39D, W46E, I55V, H60S, H60Q, Q62W, R65H, D69N, G70E, A73P, A73D, A73E, K74D, K74P, K74L, K74N, K75C, K75Q, G76T, C77A, Q79L, Q79R, Q79A, Q79G, Q79F, S80P, I85V, A101L, C108A, A109D, A109E, A109G, A109F, A109P, THIS, T111D, T111Q, T114H, T116A, T118R, T118S, S120R, P123E, N126Y, P127V, P127L, C133A, N137V, N137E, N137S, N137P, A138V, A138H, A138D, A138P, N139P, N139A, N139H, T141R, T141E, T141G, T141A, D142R, S146E, S146R, S151P, G157R, G157Q, G157N, G157L, G157A, R159Y, T161P, P173Y, P173S, N176P, N176K, C178A, K183R, Q184S, D185N, D185L, E186V, E186A, S187P, C188A, 5189T, N204C, V211W, G233E, G235Y, T245E, Q253V, Y255D, R267A, H282N, P283G, P284T, K286F, Q287Y, A288E, A288R, A288V, V291I, T295I, V297T, G311S, E315G, E315S, N317L, W318Y, T327Y, L341Y, L341V, F354Y, K363A, K363L, S367F, N369P, T370P, A380P, A380R, A380T, C382A, E383S, R385S, R385V, R385T, C391A, E402R, E402T, E402D, E402P, E402N and C408A. In some preferred implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has at least one of the following mutations: A25F, W46E, Q62W, G70E, A73P, K74N, K75C, K75Q, G76T, T114H, N137V, D142R, S146E, R159Y, P173S, N204C, Y255D, H282N, P283G and P284T. In some more preferred implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has at least one of the following mutations: A25F, W46E, Q62W, G70E, A73P, K74N, K75C, K75Q, G76T, T114H, N137V, D142R, S146E, R159Y, P173S, N204C and Y255D.

In some implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has at least one of the following mutations: W46E, Q62W, A73P, K75C, S146E, R159Y, N204C and Y255D. In some other implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has at least one of the following mutations: A25F, W46E, Q62W, G70E, A73P, K75C, T114H, N137V, D142R, S146E, R159Y and Y255D. In some implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has at least one of the following mutations: W46E, Q62W, G70E, A73P, K74N, K75Q, G76T, S146E, R159Y, P173S, Y255D, H282N, P283G and P284T. In some other implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has at least one of the following mutations: A25F, W46E, Q62W, G70E, A73P, K74N, K75Q, T114H, N137V, D142R, S146E, R159Y, P173S, Y255D, H282N, P283G and P284T.

In some specific implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has the following mutations: W46E, Q62W, A73P, K75C, S146E, R159Y, N204C and Y255D. In some other specific implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has the following mutations: A25F, W46E, Q62W, G70E, A73P, K75C, T114H, N137V, D142R, S146E, R159Y and Y255D. In some specific implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has the following mutations: W46E, Q62W, G70E, A73P, K74N, K75Q, G76T, S146E, R159Y, P173S, Y255D, H282N, P283G and P284T. In some other specific implementations, compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has the following mutations: A25F, W46E, Q62W, G70E, A73P, K74N, K75Q, T114H, N137V, D142R, S146E, R159Y, P173S, Y255D, H282N, P283G and P284T.

Specifically, the inventor has found that through introduction of a pair or a plurality of pairs of disulfide bond combinations as shown in Table 1 to the sequence of the Escherichia coli wild-type phytase as shown by SEQ ID NO: 1 or the sequence of the Escherichia mutant coli phytase as shown by SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 79 or SEQ ID NO: 99, its heat stability can be improved.

TABLE 1 Name and position of introduced disulfide bonds (amino acid position number is subject to SEQ ID NO: 1) Disulfide bond name Disulfide bond site A P34/Q174 B A56/G103 C Y57/L366 D Y61/L366 E Q82/S296 F L128/D203 G V140/E262 H T156/T191 I E165/T245 J T191/A210 K S196/V211 L A264/G312 M E315/A380 N G322/T356 O Q346/L393 P Q349/M390

In some implementations, a pair or a plurality of pairs of disulfide bonds is introduced to the wild-type or mutant Escherichia coli phytase. The disulfide bonds are selected from at least one item of (A), (B), (C), (D), (E), (J), (M) or (0), and the condition is that (C) and the (D) items are not met at the same time.

In some implementations, a pair of disulfide bonds is introduced to the wild-type or mutant Escherichia coli phytase. The disulfide bonds are selected from (A), (B), (C), (D), (E), (J), (M) or (O) items.

In some implementations, a plurality of pairs of disulfide bonds is introduced to the wild-type or mutant Escherichia coli phytase at the same time. Preferably, disulfide bonds (B)+(O), (C)+(O), (M)+(O), (B)+(D)+(O) or (D)+(M)+(O) are introduced to the wild-type or mutant Escherichia coli phytase at the same time. More preferably, disulfide bonds (B)+(O) or (C)+(O) are introduced to the wild-type or mutant Escherichia coli phytase at the same time.

For the purpose of the present invention, “introduction” does not limit any specific generation mode of the disulfide bonds. For example, “introduction” of the disulfide bonds may include replacement of an amino acid residue on a corresponding position of a phytase sequence to be introduced with the disulfide bonds with an amino acid residue capable of forming the disulfide bonds (including, for example, a cysteine residue Cys and a homocysteine residue Hcy); and insertion of an amino acid residue capable of forming the disulfide bonds on a corresponding position. Such replacement and/or insertion may be, for example, realized by a site-directed mutagenesis method well known in the art. “Introduction” also includes a condition that any one or two amino acid residues for forming the disulfide bonds are generated by natural mutation.

In order to produce a mutant modified in such a way, microbial bacteria such as Escherichia coli and fungi such as yeast (Pichia, Schizosaccharomyces pombe, etc.), filamentous fungi (such as Aspergillus niger, Aspergillus oryzae and Trichoderma reesei), and plants (such as corn, soybean and wheat) may be used as hosts for expression.

Expression and production of the mutant can be completed by general and known technologies. For example, APPLIED AND ENVIRONMENTAL MICROBIOLOGY, 2004, 3041-3046 describes expression of an Escherichia coli phytase and mutant in Escherichia coli. Journal of Biotechnology 175 (2014) 1-6 describes expression of the phytase and the mutant in Pichia. The patent application CN107353327 describes expression of the phytase and the mutant in Aspergillus niger.

In order to build the above mutation, a conventional site-directed mutation method can be used on the basis of the existing wild-type nucleotide sequence, and de novo synthesis can also be performed by a gene synthesis method. Introduction to a host cell is performed after promoter and terminator connection, and expression is performed under a proper culture condition. The method is a conventional method in the art.

“Wild-type phytase” refers to a phytase discovered in the nature and expressed by natural existing microbes, such as Escherichia coli cells.

“Mutant” or “mutant-type” refers to a polypeptide having phytase activity and including change of one or more (a plurality of) amino acid residues on one or more (a plurality of) positions, i.e., substitution, insertion and/or deletion. Substitution refers to replacement of an amino acid occupying a certain position by different amino acids. Deletion refers to removal of an amino acid occupying a certain position. Insertion refers to addition of 1-5 amino acids on and behind an adjoining position of the amino acid occupying a certain position. Mutation on the wild-type phytase also refers to amino acid substitution, insertion and/or deletion on at least one position in comparison to the wild-type phytase, and preferably refers to amino acid substitution on at least one position, such as “A25F”, i.e., phenylalanine in a 25^(th) alanine substitution position of the wild-type phytase.

“(B)+(O)”, “B+O” or “(B) item+(O) item” refers to introduction of two disulfide bonds to the sequence of the wild-type or mutant phytase, i.e., disulfide bonds are formed on two positions of the (B) item, and disulfide bonds are formed on two positions of the (O) item. Similarly, the descriptions with “+”, such as the (C) item+(O) item, (M) item+(O) item, (B) item+(D) item+(O) item and (D) item+(M) item+(O) item in the invention have similar interpretations.

“Sequence identity” is defined into percentage of amino acid residues identical to amino acid residues in a specific peptide or polypeptide sequence of a candidate sequence without regarding any conservative substitution as a part of the sequence identity after sequence comparison and gap introduction as necessary so as to obtain the maximum percentage sequence identity. Sequence comparison may be performed in various modes in the technical scope of the art, so as to determine the percentage amino acid sequence identity, for example, by using public available computer software, such as BLAST, BLAST-2, ALIGN or Megalign (DNASTAR) software. Those skilled in the art can determine to measure proper parameters of comparison, including any algorithm required for obtaining maximum comparison on the compared sequence overall length.

Based on the findings, the present application provides the following technical solution.

1. A thermostable phytase, characterized in that at least one pair of introduced disulfide bonds is included in the amino acid sequence of wild-type Escherichia coli phytase or mutant Escherichia coli phytase, the amino acid sequence of the wild-type Escherichia coli phytase is as shown by SEQ ID NO: 1, and compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on at least 1 position; and

the introduced disulfide bonds are selected from:

(A) a disulfide bond formed between an amino acid residue on a position corresponding to a 34^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 174^(th) position of SEQ ID NO: 1;

(B) a disulfide bond formed between an amino acid residue on a position corresponding to a 56^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 103^(rd) position of SEQ ID NO: 1;

(C) a disulfide bond formed between an amino acid residue on a position corresponding to a 57^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 366^(th) position of SEQ ID NO: 1;

(D) a disulfide bond formed between an amino acid residue on a position corresponding to a 61^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 366^(th) position of SEQ ID NO: 1;

(E) a disulfide bond formed between an amino acid residue on a position corresponding to an 82^(nd) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 296^(th) position of SEQ ID NO: 1;

(F) a disulfide bond formed between an amino acid residue on a position corresponding to a 128^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 203^(rd) position of SEQ ID NO: 1;

(G) a disulfide bond formed between an amino acid residue on a position corresponding to a 140^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 262^(nd) position of SEQ ID NO: 1;

(H) a disulfide bond formed between an amino acid residue on a position corresponding to a 156^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 191^(st) position of SEQ ID NO: 1;

(I) a disulfide bond formed between an amino acid residue on a position corresponding to a 165^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 245^(th) position of SEQ ID NO: 1;

(J) a disulfide bond formed between an amino acid residue on a position corresponding to a 191^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 210^(th) position of SEQ ID NO: 1;

(K) a disulfide bond formed between an amino acid residue on a position corresponding to a 196^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 211^(th) position of SEQ ID NO: 1;

(L) a disulfide bond formed between an amino acid residue on a position corresponding to a 264^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 312^(th) position of SEQ ID NO: 1;

(M) a disulfide bond formed between an amino acid residue on a position corresponding to a 315^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 380^(th) position of SEQ ID NO: 1;

(N) a disulfide bond formed between an amino acid residue on a position corresponding to a 322^(nd) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 356^(th) position of SEQ ID NO: 1;

(O) a disulfide bond formed between an amino acid residue on a position corresponding to a 346^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 393^(rd) position of SEQ ID NO: 1; and

(P) a disulfide bond formed between an amino acid residue on a position corresponding to a 349^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 390^(th) position of SEQ ID NO: 1; and

the conditions are as follows:

the (C) item and the (D) item are not met at the same time; and

the (H) item and the (J) item are not met at the same time.

2. The thermostable phytase according to implementation 1, wherein compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on at least one of the following positions: 1, 25, 30, 36, 37, 38, 39, 46, 55, 60, 62, 65, 69, 70, 73, 74, 75, 76, 77, 79, 80, 85, 101, 108, 109, 111, 114, 116, 118, 120, 123, 126, 127, 133, 137, 138, 139, 141, 142, 146, 151, 157, 159, 161, 173, 176, 178, 180, 183, 184, 185, 186, 187, 188, 189, 204, 211, 233, 235, 245, 253, 255, 267, 276, 282, 283, 284, 286, 287, 288, 291, 295, 297, 311, 315, 317, 318, 327, 341, 354, 363, 367, 369, 370, 380, 382, 383, 385, 391, 402 and 408.

3. The thermostable phytase according to implementation 2, wherein compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has at least one of the following mutations: Q1S, Q1V, Q1N, A25F, Q30K, A36K, W37F, P38Y, T39D, W46E, I55V, H60S, H60Q, Q62W, R65H, D69N, G70E, A73P, A73D, A73E, K74D, K74P, K74L, K74N, K75C, K75Q, G76T, C77A, Q79L, Q79R, Q79A, Q79G, Q79F, S80P, I85V, A101L, C108A, A109D, A109E, A109G, A109F, A109P, THIS, T111D, T111Q, T114H, T116A, T118R, T118S, S120R, P123E, N126Y, P127V, P127L, C133A, N137V, N137E, N137S, N137P, A138V, A138H, A138D, A138P, N139P, N139A, N139H, T141R, T141E, T141G, T141A, D142R, S146E, S146R, S151P, G157R, G157Q, G157N, G157L, G157A, R159Y, T161P, P173Y, P173S, N176P, N176K, C178A, K183R, Q184S, D185N, D185L, E186V, E186A, S187P, C188A, S189T, N204C, V211W, G233E, G235Y, T245E, Q253V, Y255D, R267A, H282N, P283G, P284T, K286F, Q287Y, A288E, A288R, A288V, V291I, T295I, V297T, G311S, E315G, E315S, N317L, W318Y, T327Y, L341Y, L341V, F354Y, K363A, K363L, S367F, N369P, T370P, A380P, A380R, A380T, C382A, E383S, R385S, R385V, R385T, C391A, E402R, E402T, E402D, E402P, E402N and C408A.

4. The thermostable phytase according to implementation 3, wherein compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has any combination of mutations selected from the following group: W46E, Q62W, A73P, K75C, S146E, R159Y, N204C and Y255D;

A25F, W46E, Q62W, G70E, A73P, K75C, T114H, N137V, D142R, S146E, R159Y and Y255D;

W46E, Q62W, G70E, A73P, K74N, K75Q, G76T, S146E, R159Y, P173S, Y255D, H282N, P283G and P284T; and

A25F, W46E, Q62W, G70E, A73P, K74N, K75Q, T114H, N137V, D142R, S146E, R159Y, P173S, Y255D, H282N, P283G and P284T.

5. The thermostable phytase according to implementation 4, wherein compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has any combination of mutations selected from the following group:

W46E, Q62W, A73P, K75C, S146E, R159Y, N204C and Y255D;

A25F, W46E, Q62W, G70E, A73P, K75C, T114H, N137V, D142R, S146E, R159Y and Y255D; and

W46E, Q62W, G70E, A73P, K74N, K75Q, G76T, S146E, R159Y, P173S, Y255D, H282N, P283G and P284T.

6. The thermostable phytase according to implementation 3, wherein the amino acid sequence of the mutant Escherichia coli phytase is SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 79 or SEQ ID NO: 99.

7. The thermostable phytase according to any one of implementations 1-6, wherein the disulfide bonds are selected from at least one item of (A), (B), (C), (D), (E), (J), (M) or (0), and the condition is that the (C) item and the (D) item are not met at the same time.

8. The thermostable phytase according to implementation 7, wherein the amino acid sequence meets any two, three or more items in the follow group: the (A) item, the (B) item, the (C) item, the (D) item, the (E) item, the (J) item, the (M) item and the (O) item, and the condition is that the (C) item and the (D) item are not met at the same time.

9. The thermostable phytase according to implementation 7, wherein the amino acid sequence meets the (A) item, the (B) item, the (C) item, the (E) item, the (J) item or the (M) item.

10. The thermostable phytase according to implementation 7, wherein the amino acid sequence at least meets the (D) item or the (O) item.

11. The thermostable phytase according to implementation 10, wherein the amino acid sequence meets the (D) item.

12. The thermostable phytase according to implementation 10, wherein the amino acid sequence meets the (O) item.

13. The thermostable phytase according to implementation 7, wherein the amino acid sequence meets the (B) item and the (O) item; the (C) item and the (O) item; the (D) item and the (O) item; the (M) item and the (O) item; the (B) item, the (D) item and the (O) item; or the (D) item, the (M) item and the (O) item.

14. The thermostable phytase according to implementation 13, wherein the amino acid sequence meets the (B) item and the (O) item.

15. The thermostable phytase according to implementation 13, wherein the amino acid sequence meets the (C) item and the (O) item.

16. The thermostable phytase according to implementation 13, wherein the amino acid sequence meets the (M) item and the (O) item.

17. The thermostable phytase according to implementation 13, wherein the amino acid sequence meets the (B) item, the (D) item and the (O) item.

18. The thermostable phytase according to implementation 13, wherein the amino acid sequence meets the (D) item, the (M) item and the (O) item.

19. The thermostable phytase according to any one of implementations 1-18, further including at least one pair of introduced disulfide bonds, wherein the disulfide bonds are selected from:

(i) a disulfide bond formed between an amino acid residue on a position corresponding to a 31^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 176^(th) position of SEQ ID NO: 1;

(ii) a disulfide bond formed between an amino acid residue on a position corresponding to a 31^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 177^(th) position of SEQ ID NO: 1;

(iii) a disulfide bond formed between an amino acid residue on a position corresponding to a 52^(nd) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 99^(th) position of SEQ ID NO: 1;

(iv) a disulfide bond formed between an amino acid residue on a position corresponding to a 59^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 100^(th) position of SEQ ID NO: 1;

(v) a disulfide bond formed between an amino acid residue on a position corresponding to a 91^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 46^(th) position of SEQ ID NO: 1;

(vi) a disulfide bond formed between an amino acid residue on a position corresponding to a 141^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 200^(th) position of SEQ ID NO: 1;

(vii) a disulfide bond formed between an amino acid residue on a position corresponding to a 162^(nd) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 248^(th) position of SEQ ID NO: 1; and

(viii) a disulfide bond formed between an amino acid residue on a position corresponding to a 205^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 257^(th) position of SEQ ID NO: 1; and the condition is that the (i) item and the (ii) item are not met at the same time.

20. The thermostable phytase according to implementation 1, wherein the thermostable phytase has any amino acid sequence selected from the following group: SEQ ID NOs: 4-40, SEQ ID NOs: 80-88 and SEQ ID NOs: 100-108.

21. The thermostable phytase according to any one of the above implementations, wherein the thermostable phytase is obtained through heterologous expression in a Pichia or Aspergillus niger host.

22. The thermostable phytase according to any one of the above implementations, wherein the amino acid residue capable of forming the disulfide bonds is a cysteine residue or a homocysteine residue.

23. A polynucleotide, coding the thermostable phytase according to any one of implementations 1-22.

24. The polynucleotide according to implementation 23, wherein a coding sequence of the polynucleotide is subjected to codon optimization so as to realize expression in Pichia or Aspergillus niger.

25. The polynucleotide according to implementation 23, having a nucleotide sequence as shown by any one of SEQ ID NOs: 41-77, SEQ ID NOs: 90-98 and SEQ ID NOs: 110-118.

26. A host cell, including the polynucleotide according to any one of implementations 23-25.

27. The host cell according to implementation 26, wherein the host cell is a fungus cell, a bacterium cell or a plant cell.

28. The host cell according to implementation 27, being a yeast cell or a filamentous fungus cell.

29. The host cell according to implementation 28, being a Pichia cell or an Aspergillus niger cell.

30. A method for improving heat stability of a phytase, including: changing the amino acid sequence of a phytase of interest or the coded nucleotide sequence thereof so that a disulfide bond is capable of being formed between amino acid residues on two positions of at least one item selected from the following (A) to (P) in the amino acid sequence of the phytase:

(A) a position corresponding to a 34^(th) position of SEQ ID NO: 1, and a position corresponding to a 174^(th) position of SEQ ID NO: 1;

(B) a position corresponding to a 56^(th) position of SEQ ID NO: 1, and a position corresponding to a 103^(rd) position of SEQ ID NO: 1;

(C) a position corresponding to a 57^(th) position of SEQ ID NO: 1, and a position corresponding to a 366^(th) position of SEQ ID NO: 1;

(D) a position corresponding to a 61^(st) position of SEQ ID NO: 1, and a position corresponding to a 366^(th) position of SEQ ID NO: 1;

(E) a position corresponding to an 82^(nd) position of SEQ ID NO: 1, and a position corresponding to a 296^(th) position of SEQ ID NO: 1;

(F) a position corresponding to a 128^(th) position of SEQ ID NO: 1, and a position corresponding to a 203^(rd) position of SEQ ID NO: 1;

(G) a position corresponding to a 140^(th) position of SEQ ID NO: 1, and a position corresponding to a 262^(nd) position of SEQ ID NO: 1;

(H) a position corresponding to a 156^(th) position of SEQ ID NO: 1, and a position corresponding to a 191^(st) position of SEQ ID NO: 1;

(I) a position corresponding to a 165^(th) position of SEQ ID NO: 1, and a position corresponding to a 245^(th) position of SEQ ID NO: 1;

(J) a position corresponding to a 191^(st) position of SEQ ID NO: 1, and a position corresponding to a 210^(th) position of SEQ ID NO: 1;

(K) a position corresponding to a 196^(th) position of SEQ ID NO: 1, and a position corresponding to a 211^(th) position of SEQ ID NO: 1;

(L) a position corresponding to a 264^(th) position of SEQ ID NO: 1, and a position corresponding to a 312^(th) position of SEQ ID NO: 1;

(M) a position corresponding to a 315^(th) position of SEQ ID NO: 1, and a position corresponding to a 380^(th) position of SEQ ID NO: 1;

(N) a position corresponding to a 322^(nd) position of SEQ ID NO: 1, and a position corresponding to a 356^(th) position of SEQ ID NO: 1;

(O) a position corresponding to a 346^(th) position of SEQ ID NO: 1, and a position corresponding to a 393^(rd) position of SEQ ID NO: 1; and

(P) a position corresponding to a 349^(th) position of SEQ ID NO: 1, and a position corresponding to a 390^(th) position of SEQ ID NO: 1; and

the conditions are as follows:

the (C) item and the (D) item are not selected at the same time; and

the (H) item and the (J) item are not selected at the same time.

31. The method according to implementation 30, wherein through the change, the disulfide bond is capable of being formed between the amino acid residues on two positions of at least one item selected from the (A) item, the (B) item, the (C) item, the (D) item, the (E) item, the (J) item, the (M) item or the (O) item in the amino acid sequence of the phytase of interest, and the condition is that the (C) item and the (D) item are not selected at the same time.

32. The method according to implementation 31, wherein through the change, the disulfide bond is capable of being formed between the amino acid residues on two positions of any two, three or more items selected from the following group of the amino acid sequence: the (A) item, the (B) item, the (C) item, the (D) item, the (E) item, the (J) item, the (M) item and the (O) item, and the condition is that the (C) item and the (D) item are not selected at the same time.

33. The method according to implementation 31, wherein through the change, the disulfide bond is capable of being formed between the amino acids on two positions of the (A) item, the (B) item, the (C) item, the (E) item, the (J) item or the (M) item in the amino acid sequence.

34. The method according to implementation 31, wherein through the change, the disulfide bond is capable of being formed between the amino acids on two positions of at least the (D) item or the (O) item in the amino acid sequence.

35. The method according to implementation 34, wherein through the change, the disulfide bond is capable of being formed between the amino acids on two positions of the (D) item in the amino acid sequence.

36. The method according to implementation 34, wherein through the change, the disulfide bond is capable of being formed between the amino acids on two positions of the (O) item in the amino acid sequence.

37. The method according to implementation 31, wherein through the change, the disulfide bond is capable of being formed between the amino acids on two positions respectively of the (B) item and the (O) item in the amino acid sequence.

38. The method according to implementation 31, wherein through the change, the disulfide bond is capable of being formed between the amino acids on two positions respectively of the (C) item and the (O) item in the amino acid sequence.

39. The method according to implementation 31, wherein through the change, the disulfide bond is capable of being formed between the amino acids on two positions respectively of the (M) item and the (O) item in the amino acid sequence.

40. The method according to implementation 31, wherein through the change, the disulfide bond is capable of being formed between the amino acids on two positions respectively of the (B) item, the (D) item and the (O) item in the amino acid sequence.

41. The method according to implementation 31, wherein through the change, the disulfide bond is capable of being formed between the amino acids on two positions respectively of the (D) item, the (M) item and the (O) item in the amino acid sequence.

42. The method according to any one of implementations 30-41, wherein the phytase of interest is from Escherichia coli, and has an amino acid sequence with at least 75% of sequence identity with an amino acid sequence as shown by SEQ ID NO: 1, SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 79 or SEQ ID NO: 99.

43. The method according to implementation 42, wherein the phytase of interest is wild-type Escherichia coli phytase, and preferably has an amino acid sequence as shown by SEQ ID NO: 1.

44. The method according to implementation 42, wherein the phytase of interest is mutant Escherichia coli phytase, and preferably has an amino acid sequence as shown by SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 79 or SEQ ID NO: 99.

45. The method according to any one of implementations 30-44, wherein the changed phytase has an amino acid sequence selected from the following group, or its coded nucleic acid has a nucleotide sequence coding the amino acid sequence selected from the following group: SEQ ID NOs: 4-40, SEQ ID NOs: 80-88 and SEQ ID NOs: 100-108.

46. The method according to any one of implementations 30-45, wherein the method further includes generating a phytase having the modified amino acid sequence, and putting the phytase into an environment allowing formation of disulfide bonds.

47. The method according to implementation 46, wherein the generating a phytase having the modified amino acid sequence includes expressing polynucleotide coding the phytase in a host cell.

48. The method according to implementation 47, wherein the host cell is a fungus cell, is preferably a yeast cell or a filamentous fungus cell, and is more preferably a Pichia cell or an Aspergillus niger cell.

The present invention has the following beneficial effects:

As described in the present invention, a pair or a plurality of pairs of disulfide bonds is introduced to a phytase and a mutant. Particularly, a plurality of pairs of disulfide bonds is introduced at the same time. At least one pair of disulfide bonds is introduced to the wild-type or mutant phytase in the present invention, which has its residual activity improved by about 1-8 times compared to that of the wild-type phytase. Therefore, the technical solution of the present invention can improve enzyme activity of the phytase. The phytase is significantly superior to the existing wild-type or mutant phytase especially in aspects of heat stability, steam stability and granulation stability. Its heat stability is also significantly improved compared to that of the existing engineered phytase in which disulfide bonds are introduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pPIC9K-WT plasmid profile.

FIG. 2 is a diagram showing a heat stability test result of a wild-type and disulfide bond mutant.

FIG. 3 is a diagram of a heat stability test result of an APPA-M1 and mutant.

FIG. 4 is a diagram of a heat stability test result of an APPAan-WT and mutant.

FIG. 5 is a diagram of a heat stability test result of an APPA-M2 and mutant.

DETAILED DESCRIPTION

Embodiment 1 Disulfide bond mutant building, and wild-type and mutant expression in Pichia

A 3D structure of an Escherichia coli phytase has been published (referring to Lim D et al, Nat Struct Biol. 2000, 7(2):108-13), and with reference to a 3D structure document PDB ID 1DKO, disulfide bonds as described in the table below were designed and introduced.

Disulfide bond name Disulfide bond site A P34C/Q174C B A56C/G103C C Y57C/L366C D Y61C/L366C E Q82C/S296C F L128C/D203C G V140C/E262C H T156C/T191C I E165C/T245C J T191C/A210C K S196C/V211C L A264C/G312C M E315C/A380C N G322C/T356C O Q346C/L393C P Q349C/M390C Q T33C/L170C R I55C/A99C S A268C/N309C T I85C/G97C U P123C/T130C V A226C/M360C W W243C/P324C X W347C/M390C Y I348C/F396C

The amino acid sequence of a wild-type phytase was as shown by SEQ ID NO: 1. A nucleotide sequence thereof expressed in the Pichia was as shown by SEQ ID NO: 78. An expression vector was pPIC9K. A Saccharomyces cerevisiae Alpha factor was used as a signal peptide. An expression plasmid pPIC9K-WT of the wild-type phytase was as shown in FIG. 1.

In order to build the mutant in the above table, primers were respectively designed for Polymerase Chain Reaction (PCR). The primers were as shown by the table below.

Disul- Disul- fide fide bond bond name site Mutation primer A P34/ A-F1 GATGTCACCtgtGACGCTTGGCCAACCTGGA- Q174C F2 AACTTCCCAtgtTCAAACTTGTGCTTGAAG A-R1 CCAAGCGTCacaGGTGACATCTTGCATAAG A-R2 CAAGTTTGAacaTGGGAAGTTAAGAACTCT B A56/ B-F1 GAGCTCATTtgtTACTTGGGTCACTACCAA G103C B-F2 TTCGCCGCCtgtCTTGCTCCTGACTGTGCC B-R1 ACCCAAGTAacaAATGAGCTCACCACCTCT B-R2 AGGAGCAAGacaGGCGGCGAAGGCTTCACC C T57C/ C-F1 CTCATTGCTtgtTTGGGTCACTACCAAAGAC- L366C F2 AAGACTCCAtgtTCTTTGAACACGCCTCCA C-R1 GTGACCCAAacaAGCAATGAGCTCACCACC C-R2 GTTCAAAGAacaTGGAGTCTTTGTCTCTCAT D T61C/ D-F1 TTGGGTCAtgtCAAAGACAGCGTCTTGTT L36C D-F2 AAGACTCCAtgtTCTTTGAACACGCCTCCA D-R1 CTGTCTttgacaGTGACCCAAGTAAGCAAT D-R2 GTTCAAAGAacaTGGAGTCTTGTCTCTCAT E Q82C/ E-F1 CAATCTGGTtgtGTAGCTATTATTGCTGAC S296C E-F2 TTGCCCACTtgtGTCTTGTTCATTGCCGGT E-R1 AATAGCTACacaACCAGATTGTGGACAACC E-R2 GAACAAGACacaAGTGGGCAAGGTAACACC F L128C/ F-F1 TTCAACCCTtgtAAGACTGGTGTTTGCCA D203C F-F2 GTCTCCGCCtgtAACGTCTCTTTGACCGGT F-R1 ACCAGTCTTacaAGGGTTGAACAATGGATC F-R2 AGAGACGTTacaGGCGGAGACCTTCAACTC G V140C/ G-F1 aacGCTAACtgtACTGACGCTATCTTGTCC E262C G-F2 AGAACTCCAtgtGTTGCTAGATCCAGAGCC G-R1 AGCGTCAGTacaGTTAGCGTTGTCCAATTG G-R2 TCTAGCAAcacATGGAGTTCTCTGCAGCAA H T156C/ H-F1 GCTGACTTCtgtGGTCACAGACAGACTGCC T191C H-F2 TGTTCCTTGtgtCAAGCATTACCATCTGAGH- R1 tctGTGACCacaGAAGTCAGCAATGGATCC H-R2TAATGCTTGacaCAAGGAACAGGATTCGTC I E165C/ I-F1 GCCTTCAGAtgtTTGGAAAGAGTTCTTAAC T245C I-F2 CAATGGAACtgtTTGTTGTCCTTGCACAAC I-R1 TCTTTCCAAacaTCTGAAGGCAGTCTGTCT I-R2 GGACAACAAacaGTTCCATTGGTGAGAGTC J T191C/ J-F1 TGTTCCTTGtgtCAAGCATTACCATCTGAG A2I0C J-F2 TTGACCGGTtgtGTCAGCTTGGCTTCCATG J-R1 TAATGCTTGacaCAAGGAACAGGATTCGTC J-R2 CAAGCTGACacaACCGGTCAAAGAGACGT K S196C/ K-F1 GCATTACCAtgtGAGTTGAAGGTCTCCGCC V211C K-F2 ACCGGTGCTtgtAGCTTGGCTTCCATGTTG K-R1 CTTCAACTCacaTGGTAATGCTTGAGTCAA K-R2 AGCCAAGCTacaAGCACCGGTCAAAGAGAC L A264C/ T-F1 CCAGAGgTTtgtAGATCCAGAGCCACCCCA G312C T-F2 AATCTCGGCtgtGCTTTGGAGTTGAACTGG T-R1 TCTGGATCTacaAAcCTCTGGAGTTCTCTG T-R2 CTCCAAAGCacaGCCGAGATTTGCCAAGTT M E315C/ M-F1 GGTGCTTTGtgtTTGAACTGGACTCTTCCT A380C M-F2 TTGACCTTGtgtGGATGTGAAGAGAGAAAT M-R1 CCAGTTCAAacaCAAAGCACCGCCGAGATT M-R2 TTCACATCCacaCAAGGTCAATTTGACTTC N G322C/ N-F1 ACTCTTCCTtgtCAACCTGATAACACTCCA T356C N-F2 GTCTTCCAAtgtTTGCAGCAGATGAGAGAC N-R1 ATCAGGTTGacaAGGAAGAGTCCAGTTCAA N-R2 CTGCTGCAAacaTTGGAAGACCAACGAAAC O Q346C/ O-F1 GATAACTCTtgtTGGATTCAGGTTTCGTTG L393C O-F2 ATGTGTTCCtgtGCTGGTTTCACTCAAATC O-R1 CTGAATCCAacaAGAGTTATCAGATAGTCT O-R2 GAAACCAGCacaGGAACACATACCCTGAGC P Q349C/ P-F1 CAATGGATTIgtGTTTCGTTGOTCTTCCAA M390C P-F2 GCTCAGGGTtgtTGTTCCTTGGCTGGTTTC P-R1 CAACGAAACacaAATCCATTGAGAGTTATC P-R2 CAAGGAACAacaACCCTGAGCATTTCTCTC Q T33C/ Q-F1 CAAGATGTCtgtCCAGACGCTTGGCCAACC L170C Q-F2 GAAAGAGTTtgtAACTTCCCACAAtcaAAC Q-R1 AGCGTCTGGacaGACATCTTGCATAAGTTG Q-R2 TGGGAAGTTacaAACTCTTTCCAACTCTCT R I55C/ R-F1 GGTGAGCTCtgtGCTTACTTGGGTCACTAC A99C R-F2 ACAGGTGAAtgtTTCGCCGCCGGTCTTGCT R-R1 CAAGTAAGCacaGAGCTCACCACCTCTAGG R-R2 GGCGGCGAAacaTTCACCTGTCTTACGGGT S A268C/ S-F1 AGATCCAGAtgTACCCCATTGTTGGACTTG N309C S-F2 AACTTGGCATgtCTCGGCGGTGCTTTGGAG S-R1 CAATGGGGTacaTCTGGATCTAGCAAcCTC S-R2 ACCGCCGAGacaTGCCAAGTTAGTATCGTG T I85C/ T-F1 CAAGTAGCTtgtATTGCTGACGTCGACGAA G97C T-F2 CGTAAGACACgtGAAGCCTTCGCCGCCGGT T-RTGTCAGCAATacaAGCTACTTGACCAGATTG T-R2 GAAGGCTTCACaTGTCTTACGGGTTCTTTC U P123C/ U-F1 TCTCCAGATtgtTTGTTCAACCCTTTGAAG T130C U-F2 CCTTTGAAGtgtGGTGTTTGCCAATTGGAC U-RTGTTGAACAAacaATCTGGAGAAGAAGTGTC U-R2 GCAAACACCacaCTTCAAAGGGTTGAACAA V A226C/ V-F1 CTGCAACAAtgtCAAGGTATGCCTGAGCCA M360C V-F2 TTGCAGCAGtgtAGAGACAAGACTCCACTG V-R1 CATACCTTGAcaTTGTTGCAGAAGAAAGAT V-R2 CTTGTCTCTacaCTGCTGCAAAGTTTGGAA W W243C/ W-F1 TCTCACCAATGtAACACCTTGTTCJTCCTTG P324C W-F2 CCTGGTCAAtgtGATAACACTCCACCAGGT W-R1 CAAGGTGTTacaTTGGTGAGAGTCGGTGAT W-R2 AGTGTTATCacaTTGACCAGGAAGAGTCCA X W347C/ X-F1 AACTCTCAAtgTATTCAGGTTTCGTTGGTC M390C X-F2 GCTCAGGGTgtTTGTTCCTTGGCTGGTTTC X-R1 AACCTGAATaCATTGAGAGTTATCAGATAG X-R2 CAAGGAACAacaACCCTGAGCATTTCTCTC Y I348C/ Y-F1 TCTCAATGGtgtCAGGTTTCGTTGGTCTTC F396C Y-F2 TTGGCTGGTtgtACTCAAATCGTTAACGAA Y-R1 CGAAACCTGacaCCATTGAGAGTTATCAGA Y-R2 GATTTGAGTacaACCAGCCAAGGAACACAT

In order to introduce 25 pairs of disulfide bonds of A-Y, plasmid pPIC9K-WT was used as a template. F1/R2 and F2/R1 were used as introduction pairs. Two PCR amplification reactions were respectively performed. The amplification reaction was completed by a Phusion® High-Fidelity DNA polymerase (New England Biolabs, article number: M0530L). Setting was performed with reference to its manual. After the amplification was completed, a Dpnl endonuclease (New England Biolabs) was added for digesting the template. Then, Gibson Assembly® Master Mix Kit (New England Biolabs, article number: E2611) was used for fragment recombination. Through sequencing, successful building of the mutant plasmid was confirmed. The mutant plasmid was respectively named as pPIC9K-A to pPIC9K-Y according to the disulfide bond name in the above table.

In order to express the phytase and the mutant, the Pichia GS115 and the plasmid were operated with reference to Pichia expression kit (Invitrogen) manual. Specifically, after a Pichia GS115 strain was subjected to plate culture at 30° C. for 48 h by a YPD culture medium (1% of yeast extracts, 2% of protein, 2% of glucose and 1.5% of agar), monoclone was selected into a 4 mL YPD liquid culture medium (1% of yeast extracts, 2% of protein, and 2% of glucose), cultured at 30° C. and 200 rpm for 12 h, transferred to an Erlenmeyer flask with 30 mL YPD liquid culture medium, and cultured at 30° C. and 220 rpm for 4-5 h. After an OD600 value was detected to be in a range of 1.1-1.3, culture solution was centrifuged at 4° C. and 9,000 rpm for 2 min. 4 mL of thalli were respectively collected into sterile EP tubes. Supernatants were slightly abandoned. After residual supernatants were thoroughly absorbed by sterile filter paper, 1 mL of precooled sterile water was used for resuspending the thalli. Centrifugation at 4° C. and 9,000 rpm was performed for 2 min. Supernatants were abandoned. The above steps were repeated. The thalli were resuspended by 1 mL of precooled sorbitol (1 mol/L). Centrifugation at 4° C. and 9,000 rpm was performed for 2 min. Supernatants were abandoned. The thalli were resuspended by 100-150 μl of precooled sorbitol (1 mol/L). Hereto, preparation of competent cells was completed. The expression plasmid pPIC9K-WT and the other 25 disulfide bond mutants were linearized by BglII. Linearized fragments were purified and recovered and were then converted into the Pichia GS115 competent cells by an electroporation method. A mixture was uniformly coated onto an MD plate to be subjected to 30° C. reverse culture for 2-3 d. All bacterial colonies on the plate were washed down by sterile water and were then coated onto a YPD (0.5 mg/mL-8 mg/mL) plate containing different concentrations of geneticin for screening multicopy converters. Pichia recombination strains were obtained through screening on the MD plate, and were named as APPA-WT and APPA-A, APPA-B, APPA-C, APPA-D, APPA-E, APPA-F, APPA-G, APPA-H, APPA-I, APPA-J, APPA-K, APPA-L, APPA-M, APPA-N, APPA-O, APPA-P, APPA-Q, APPA-R, APPA-S, APPA-T, APPA-U, APPA-V, APPA-W, and APPA-X and APPA-Y The above clones obtained through screening were respectively transferred into a BMGY culture medium to be cultured for 24 h in an oscillation table concentrator at 30° C. and 250 rpm, and were then transferred into a BMMY culture medium. Conditions of 30° C. and 250 rpm were maintained. 0.5% of methanol was added every day. After 120 h of induction expression, 9000-12000 rpm centrifugation was performed for 10 min so as to remove thalli. A fermentation supernatant containing a phytase APPA-WT and its 25 mutants was obtained. An SD S-PAGE result showed that three mutants of APPA-S, APPA-X and APPA-Y were not expressed, and the other 22 mutants were expressed.

Embodiment 2 Heat Stability Determination

Phytase activity determination conforms to GB/T 18634-2009. 23 samples in Embodiment 1 were diluted to 100 U/mL. 9 mL of water was taken in a 25 mL colorimetric tube to be respectively preheated in an 80° C. constant-temperature water bath. 1 mL of each enzyme sample was sucked by a pipette and were added fast into each corresponding test tube. Fast mixing and blending was performed by a mixing and blending device, followed by standing for 3 min. The temperature was fast cooled to a room temperature. Dilution was performed by water. Residual activity of each sample was determined. Therefore, the enzyme activity residue rates (the enzyme activity before heat treatment was set to be 100%) at different treatment temperatures were calculated. The heat stability data was as shown in FIG. 2. Some mutants showed good heat stability. APPA-B, APPA-C, APPA-D, APPA-M, APPA-O and APPA-P had the best performance, with the residual activity improved by about 20-25%, or about 2-3 times, compared with that of APPA. The above results showed that the introduction of the disulfide bonds had significant influence on the mutants. Some mutations even caused normal expression incapability, such as APPA-S, APPA-X and APPA-Y. Additionally, the introduction of some specified disulfide bonds may cause stability reduction. For example, the two mutants, such as APPA-Q and APPA-V caused heat stability significantly lower than that of the wild type. There were also some introduced disulfide bonds beneficial to the enzyme structure stability, such as APPA-A to APPA-P which can improve the heat resistance capability of the wild type.

Embodiment 3 Introduction of Disulfide Bonds on Basis of Mutant and Determination of its Stability

Nov9X is a mutant with good heat resistance obtained through mutation screening by the wild-type phytase (as described in U.S. Pat. No. 7,432,098). 8 mutations were introduced on the basis of the wild type. A specific sequence was as shown by SEQ ID NO: 2. Other mutations were continuously introduced on the basis of the Nov9X sequence. The sequence became one as shown in SEQ ID NO: 3, and the mutant was named as APPA-M1. Its heat stability can be further improved. In order to test whether the disulfide bond mutant described in Embodiment 1 can achieve a function on the phytase mutant or not, the stability was further improved. The disulfide bonds D and O and disulfide bond combinations B+O, C+O, D+O, M+O, B+D+O and D+M+O were introduced on the basis of APPA-M1 according to a method in Embodiment 1. Each mutant was respectively named as APPA-M1-D, APPA-M1-O, APPA-M1-BO, APPA-M1-CO, APPA-M1-DO, APPA-M1-MO, APPA-M1-BDO and APPA-M1-DMO. Meanwhile, according to description in US20170240872 and US20130017185, two best mutant disulfide bonds in each embodiment were introduced on the basis of APPA-M1, and the mutants were respectively named as US20170240872-A, US20170240872-B, US20130017185-B and US20130017185-C. Each mutant was expressed in Pichia. Then, heat stability was determined according to a method in Embodiment 2. A different parameter was incubation for 3 min at 85° C. The results were as shown in FIG. 3. The disulfide bonds D and O both further improved the heat stability of the mutant, and the improvement brought by the disulfide bond 0 on the stability of the APPA-M1 mutant was greater than the improvement brought on the wild-type APPA-WT. The disulfide bond O improved the stability of the APPA-M1 mutant by up to 35.5%. Disulfide bond combined introduced mutants APPA-M1-CO and APPA-M1-DO showed stability similar to APPA-M1-O. Other combination mutants, such as APPA-M1-BO, APPA-M1-MO, APPA-M1-BDO and APPA-M1-DMO had higher stability. The residual activity of APPA-M1-BO with the best stability can reach 77.2%, and was improved by about 1-1.5 times compared with that of APPA-M1, the significant heat resistance characteristic was realized, and APPA-M1-BO was predicted to have good performance in feed granulation. The above results showed that the proper combination can create more stable mutants. The stability of US20170240872-A and US20170240872-B determined by this method was improved by 1.1-8.7% compared with that of APPA-M1. The stability of US20130017185-B and US20130017185-C was improved by 5.0-14.8%. It can be seen that the introduction of the disulfide bonds provided by the inventor showed more effective results.

Embodiment 4 Expression of Escherichia coli Wild-Type Phytase in Aspergillus niger and Disulfide Bond Introduced Mutant Phytase

According to description of patent application CN107353327, the Escherichia coli phytase wild type (SEQ ID NO: 1) and mutants (A to P) were expressed. The wild-type phytase was named as APPAan-WT. Each mutant was named according to APPAan-A to APPAan-P. After shake flask supernatant was obtained, heat stability determination was performed according to description in Embodiment 2. A different parameter was incubation for 3 min at 85° C. The experiment results were as shown in FIG. 4. It is found that the wild-type phytase expressed in Aspergillus niger showed more significant stability than that expressed in Pichia probably because they had different glycosylation states. Through experiments, it is also found that the mutant APPAan-P cannot be expressed, APPAan-G expressed stability similar to WT, and the stability of APPAan-H was significantly reduced. The other 13 mutants all expressed significant stability performance improvement, and all expressed improvement of 5% to 20.5%, which was different from that of mutants expressed in Pichia. The above results showed that 16 mutants expressed in at least one host cell, and showed more excellent stability than the wild type. Through the proper introduction of the disulfide bond combination, mutants with higher stability can be obtained.

Embodiment 5 Stability Test Results after Introduction of Disulfide Bonds to Phytase Mutants

Nov9X is a mutant with good heat resistance obtained through mutation screening by the wild-type phytase (as described in U.S. Pat. No. 7,432,098). 8 mutations were introduced on the basis of the wild type. A specific sequence was as shown by SEQ ID NO: 2. On the basis of a Nov9X sequence, a glycosylation site is continuously introduced according to a literature report (Improving specific activity and thermostability of Escherichia coli phytase by structure-based rational design). The sequence became one as shown in SEQ ID NO: 79, and the mutant was named as APPA-M2. Its heat stability can be further improved. In order to test whether the disulfide bond mutant described in Embodiment 1 can achieve a function on the phytase mutant APPA-M2 or not, the stability was further improved. The disulfide bonds B, C, D, M and O and disulfide bond combinations B+O, D+O, M+O and C+O were introduced on the basis of APPA-M2 according to a method in Embodiment 1. Each mutant was respectively named as APPA-M2-B, APPA-M2-C, APPA-M2-D, APPA-M2-M, APPA-M2-O, APPA-M2-BO, APPA-M2-DO, APPA-M2-MO and APPA-M2-CCX. The amino acid sequence of each mutant was as shown by SEQ ID Nos: 80-88. Corresponding nucleotide sequences were as shown by SEQ ID NOs: 90-98. Each mutant was expressed in Aspergillus niger. Then, heat stability was determined according to a method in Embodiment 2. The results were as shown in FIG. 5. The disulfide bonds adopted in the experiment all significantly improved the heat stability of the mutant. The effect of the combined disulfide bonds C+O was best. The residual activity after heat resistance treatment can reach 84.5%. The significant heat resistance characteristic was realized. It was predicted to have good performance in feed granulation. The above results showed that the proper combination can create more stable mutants. The above results showed that the introduction of the disulfide bonds provided by the inventor still showed very effective results on the phytase mutant sequence.

The inventor also tried continuously introducing a glycosylation site on the basis of the Nov9X sequence. Its sequence was as shown by SEQ ID NO: 99. The mutant was named as APPA-M3. Its heat stability can also be improved. Disulfide bonds C, C, D, M and O and disulfide bond combinations B+O, D+O, M+O, and C+O were introduced on the basis of APPA-M3 according to the method in Embodiment 1. Each mutant was respectively named as APPA-M3-B, APPA-M3-C, APPA-M3-D, APPA-M3-M, APPA-M3-O, APPA-M3-BO, APPA-M3-DO, APPA-M3-MO and APPA-M3-CCX. The amino acid sequence of each mutant was as shown by SEQ ID Nos: 100-108. Corresponding nucleotide sequences were as shown by SEQ ID NOs: 110-118. Each mutant was expressed in Aspergillus niger. Then, heat stability was determined according to the method in Embodiment 2. It was found that the disulfide bond introduced mutants achieve a good heat resistance characteristic like the disulfide bond introduced AMMA-M2 mutants. Identically, it was predicted to have good performance in feed granulation.

The modifications and variations of the method according to the present invention are obvious to a person skilled in the art, and do not deviate from the scope of the invention. Although the present invention is described with reference to particular embodiments, it should be understood that the invention requiring protection is not improperly limited to these particular exemplary embodiments. In fact, the various mutational modifications of wild-type phytase that are apparent to a person skilled in the art to achieve the technical effects of the present invention are covered by the claims. 

1. A thermostable phytase, wherein at least one pair of introduced disulfide bonds is comprised in the amino acid sequence of wild-type Escherichia coli phytase or mutant Escherichia coli phytase, the amino acid sequence of the wild-type Escherichia coli phytase is as shown by SEQ ID NO: 1, and compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on at least 1 position; and the introduced disulfide bonds are selected from: (A) a disulfide bond formed between an amino acid residue on a position corresponding to a 34^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 174^(th) position of SEQ ID NO: 1; (B) a disulfide bond formed between an amino acid residue on a position corresponding to a 56^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 103^(rd) position of SEQ ID NO: 1; (C) a disulfide bond formed between an amino acid residue on a position corresponding to a 57^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 366^(th) position of SEQ ID NO: 1; (D) a disulfide bond formed between an amino acid residue on a position corresponding to a 61^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 366^(th) position of SEQ ID NO: 1; (E) a disulfide bond formed between an amino acid residue on a position corresponding to an 82^(nd) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 296^(th) position of SEQ ID NO: 1; (F) a disulfide bond formed between an amino acid residue on a position corresponding to a 128^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 203^(rd) position of SEQ ID NO: 1; (G) a disulfide bond formed between an amino acid residue on a position corresponding to a 140^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 262^(nd) position of SEQ ID NO: 1; (H) a disulfide bond formed between an amino acid residue on a position corresponding to a 156^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 191^(st) position of SEQ ID NO: 1; (I) a disulfide bond formed between an amino acid residue on a position corresponding to a 165^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 245^(th) position of SEQ ID NO: 1; (J) a disulfide bond formed between an amino acid residue on a position corresponding to a 191^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 210^(th) position of SEQ ID NO: 1; (K) a disulfide bond formed between an amino acid residue on a position corresponding to a 196^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 211^(th) position of SEQ ID NO: 1; (L) a disulfide bond formed between an amino acid residue on a position corresponding to a 264^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 312^(th) position of SEQ ID NO: 1; (M) a disulfide bond formed between an amino acid residue on a position corresponding to a 315^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 380^(th) position of SEQ ID NO: 1; (N) a disulfide bond formed between an amino acid residue on a position corresponding to a 322^(nd) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 356^(th) position of SEQ ID NO: 1; (O) a disulfide bond formed between an amino acid residue on a position corresponding to a 346^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 393^(rd) position of SEQ ID NO: 1; and (P) a disulfide bond formed between an amino acid residue on a position corresponding to a 349^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 390^(th) position of SEQ ID NO: 1; and the conditions are as follows: the (C) item and the (D) item are not met at the same time; and the (H) item and the (J) item are not met at the same time.
 2. The thermostable phytase according to claim 1, wherein compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has a mutation on at least one of the following positions: 1, 25, 30, 36, 37, 38, 39, 46, 55, 60, 62, 65, 69, 70, 73, 74, 75, 76, 77, 79, 80, 85, 101, 108, 109, 111, 114, 116, 118, 120, 123, 126, 127, 133, 137, 138, 139, 141, 142, 146, 151, 157, 159, 161, 173, 176, 178, 180, 183, 184, 185, 186, 187, 188, 189, 204, 211, 233, 235, 245, 253, 255, 267, 276, 282, 283, 284, 286, 287, 288, 291, 295, 297, 311, 315, 317, 318, 327, 341, 354, 363, 367, 369, 370, 380, 382, 383, 385, 391, 402 and
 408. 3. The thermostable phytase according to claim 2, wherein compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has at least one of the following mutations: Q1S, Q1V, Q1N, A25F, Q30K, A36K, W37F, P38Y, T39D, W46E, I55V, H60S, H60Q, Q62W, R65H, D69N, G70E, A73P, A73D, A73E, K74D, K74P, K74L, K74N, K75C, K75Q, G76T, C77A, Q79L, Q79R, Q79A, Q79G, Q79F, S80P, I85V, A101L, C108A, A109D, A109E, A109G, A109F, A109P, THIS, T111D, T111Q, T114H, T116A, T118R, T118S, S120R, P123E, N126Y, P127V, P127L, C133A, N137V, N137E, N137S, N137P, A138V, A138H, A138D, A138P, N139P, N139A, N139H, T141R, T141E, T141G, T141A, D142R, S146E, S146R, S151P, G157R, G157Q, G157N, G157L, G157A, R159Y, T161P, P173Y, P173S, N176P, N176K, C178A, K183R, Q184S, D185N, D185L, E186V, E186A, S187P, C188A, S189T, N204C, V211W, G233E, G235Y, T245E, Q253V, Y255D, R267A, H282N, P283G, P284T, K286F, Q287Y, A288E, A288R, A288V, V291I, T295I, V297T, G311S, E315G, E315S, N317L, W318Y, T327Y, L341Y, L341V, F354Y, K363A, K363L, S367F, N369P, T370P, A380P, A380R, A380T, C382A, E383S, R385S, R385V, R385T, C391A, E402R, E402T, E402D, E402P, E402N and C408A.
 4. The thermostable phytase according to claim 3, wherein compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has any combination of mutations selected from the following group: W46E, Q62W, A73P, K75C, S146E, R159Y, N204C and Y255D; A25F, W46E, Q62W, G70E, A73P, K75C, T114H, N137V, D142R, S146E, R159Y and Y255D; W46E, Q62W, G70E, A73P, K74N, K75Q, G76T, S146E, R159Y, P173S, Y255D, H282N, P283G and P284T; and A25F, W46E, Q62W, G70E, A73P, K74N, K75Q, T114H, N137V, D142R, S146E, R159Y, P173S, Y255D, H282N, P283G and P284T.
 5. The thermostable phytase according to claim 4, wherein compared to the wild-type Escherichia coli phytase as shown by SEQ ID NO: 1, the mutant Escherichia coli phytase has any combination of mutations selected from the following group: W46E, Q62W, A73P, K75C, S146E, R159Y, N204C and Y255D; A25F, W46E, Q62W, G70E, A73P, K75C, T114H, N137V, D142R, S146E, R159Y and Y255D; and W46E, Q62W, G70E, A73P, K74N, K75Q, G76T, S146E, R159Y, P173S, Y255D, H282N, P283G and P284T.
 6. The thermostable phytase according to claim 3, wherein the amino acid sequence of the mutant Escherichia coli phytase is SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 79 or SEQ ID NO:
 99. 7. The thermostable phytase according to claim 1, wherein the disulfide bonds are selected from at least one item of (A), (B), (C), (D), (E), (J), (M) or (O), and the condition is that the (C) item and the (D) item are not met at the same time.
 8. The thermostable phytase according to claim 7, wherein the amino acid sequence meets any two, three or more items in the follow group: the (A) item, the (B) item, the (C) item, the (D) item, the (E) item, the (J) item, the (M) item and the (O) item, and the condition is that the (C) item and the (D) item are not met at the same time.
 9. The thermostable phytase according to claim 7, wherein the amino acid sequence meets the (A) item, the (B) item, the (C) item, the (E) item, the (J) item or the (M) item.
 10. The thermostable phytase according to claim 7, wherein the amino acid sequence at least meets the (D) item or the (O) item.
 11. The thermostable phytase according to claim 10, wherein the amino acid sequence meets the (D) item.
 12. The thermostable phytase according to claim 10, wherein the amino acid sequence meets the (O) item.
 13. The thermostable phytase according to claim 7, wherein the amino acid sequence meets the (B) item and the (O) item; the (C) item and the (O) item; the (D) item and the (O) item; the (M) item and the (O) item; the (B) item, the (D) item and the (O) item; or the (D) item, the (M) item and the (O) item.
 14. The thermostable phytase according to claim 13, wherein the amino acid sequence meets the (B) item and the (O) item.
 15. The thermostable phytase according to claim 13, wherein the amino acid sequence meets the (C) item and the (O) item.
 16. The thermostable phytase according to claim 13, wherein the amino acid sequence meets the (M) item and the (O) item.
 17. The thermostable phytase according to claim 13, wherein the amino acid sequence meets the (B) item, the (D) item and the (O) item.
 18. The thermostable phytase according to claim 13, wherein the amino acid sequence meets the (D) item, the (M) item and the (O) item.
 19. The thermostable phytase according to claim 1, further comprising at least one pair of introduced disulfide bonds, wherein the disulfide bonds are selected from: (i) a disulfide bond formed between an amino acid residue on a position corresponding to a 31^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 176^(th) position of SEQ ID NO: 1; (ii) a disulfide bond formed between an amino acid residue on a position corresponding to a 31^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 177^(th) position of SEQ ID NO: 1; (iii) a disulfide bond formed between an amino acid residue on a position corresponding to a 52^(nd) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 99^(th) position of SEQ ID NO: 1; (iv) a disulfide bond formed between an amino acid residue on a position corresponding to a 59^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 100^(th) position of SEQ ID NO: 1; (v) a disulfide bond formed between an amino acid residue on a position corresponding to a 91^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 46^(th) position of SEQ ID NO: 1; (vi) a disulfide bond formed between an amino acid residue on a position corresponding to a 141^(st) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 200^(th) position of SEQ ID NO: 1; (vii) a disulfide bond formed between an amino acid residue on a position corresponding to a 162^(nd) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 248^(th) position of SEQ ID NO: 1; and (viii) a disulfide bond formed between an amino acid residue on a position corresponding to a 205^(th) position of SEQ ID NO: 1 and an amino acid residue on a position corresponding to a 257^(th) position of SEQ ID NO: 1; and the condition is that the (i) item and the (ii) item are not met at the same time.
 20. The thermostable phytase according to claim 1, wherein the thermostable phytase has any one amino acid sequence selected from the following group: SEQ ID NOs: 4-40, SEQ ID NOs: 80-88 and SEQ ID NOs: 100-108.
 21. The thermostable phytase according to claim 1, wherein the thermostable phytase is obtained through heterologous expression in a Pichia or Aspergillus niger host.
 22. The thermostable phytase according to claim 1, wherein the amino acid residue capable of forming the disulfide bonds is a cysteine residue or a homocysteine residue.
 23. A polynucleotide, coding the thermostable phytase according to claim
 1. 24. The polynucleotide according to claim 23, wherein a coding sequence of the polynucleotide is subjected to codon optimization so as to realize expression in Pichia or Aspergillus niger.
 25. The polynucleotide according to claim 23, having a nucleotide sequence as shown by any one of SEQ ID NOs: 41-77, SEQ ID NOs: 90-98 and SEQ ID NOs: 100-118.
 26. A host cell, comprising the polynucleotide according to claim
 23. 27. The host cell according to claim 26, wherein the host cell is a fungus cell, a bacterium cell or a plant cell.
 28. The host cell according to claim 27, being a yeast cell or a filamentous fungus cell.
 29. The host cell according to claim 28, being a Pichia cell or an Aspergillus niger cell. 30.-48. (canceled) 